In the past, various methods have been employed to produce anti-static woven fabrics suitable for flexible intermediate bulk containers (FIBC) or clean room garments. FIBCs are used in the packaging and transportation of dry substances such as metal ores, chemicals, foodstuffs and powders. They are designed to be handled with standard fork-lifts and typically hold from 500 to 4400 pounds of material. Common dimensions include 35 inch and 41 inch square cylinders. A common hazard of FIBCs is electrostatic discharge (ESD). ESD hazard ranges from personnel nuisance shocks to sparks capable of igniting explosive mixtures of dust or flammable gases. As a result it is necessary to eliminate ESD from flexible intermediate bulk containers in certain applications.
Clean room garments are worn by operators working in clean room environments, typically for the manufacture of semi-conductor electronic components. Often the semi-conductors are sensitive to electrostatic discharges and are damaged when subjected to ESD. The result is that clean room garments must in many cases be free of ESD, just as FIBCs must be.
Some of the textile fabrics used in FIBC and clean room garments include polypropylene and Tyvek(copyright). Polypropylene is particularly favored for FIBCs due to its inertness, strength and low cost. FIBCs made from woven polypropylene are disclosed in U.S. Pat. No. 5,071,699 to Pappas that is incorporated by reference herein.
FIBCs are either coated or uncoated. Uncoated FIBCs are breathable and allow transmission of moisture through the fabric. Coated FIBCs can restrict transmission of moisture; prevent dust escaping as well as having other special properties. For example, when ultraviolet light resistance is desired, a UV stabilizing coating is used. As an alternate, threads and yarns can be coated with a UV stabilizer before weaving into fabric.
Control of ESD from fabrics can be either conductive or dissipative. Conductive refers to the electrical conduction of any accumulated charge, to an electrical ground. Dissipative refers to the dissipation of static electricity through electrostatic discharges including corona discharges, spark discharges, brush discharges or propagating brush discharges. Spark, brush and propagating brush discharges can create incendiary discharges in many common flammable atmospheres. In contrast the corona discharges are generally below incendiary discharge energy levels.
Conductive fabrics require an electrically sufficient connection to a ground point. These fabrics function by draining an accumulating electrical charge to the ground. Any disruption in the ground connection disables their ESD control ability. Additionally, fabrication of containers formed of conductive fabrics requires specialized construction techniques to ensure all conductive surfaces are electrically connected together for a ground source.
In contrast, dissipative fabrics rely on the fabric, alone or in conjunction with an anti-static coating, to discharge charges at levels below those that cause damage or create a spark capable of igniting flammable material (for example by corona discharge). Examples of dissipative fabrics are disclosed in U.S. Pat. No. 5,512,355 to Fuson and assigned to E. I du Pont and U.S. Patents assigned to Linq Industrial Fabrics, including U.S. Pat. No. 5,478,154 to Pappas et al, U.S. Pat. No. 5,679,449 to Ebadat et al., U.S. Pat. No. 6,112,772 to Ebadat et al.
The fabrics disclosed in U.S. Pat. No. 5,512,355 comprise polypropylene yarns interwoven with sheath-core filament yarns. The sheath-core filament yarns further comprise semi-conductor carbon black or graphite containing core and a non-conducting sheath. The filaments are interlaced in the fabric at between xc2xc and 2 inch intervals. In a preferred embodiment, the filaments are crimped so that stretching of the sheath-core yarn does not break the electrical continuity of the semi-conductor core. A noted disadvantage of sheath-core filaments is the relatively high cost of resultant yarns.
The fabrics disclosed (but not claimed) in the Linq Industries assigned patents also comprise sheath-core yarns interwoven with non-conductive yarns or superimposed over non-conductive yarns. Such fabrics are identified as xe2x80x9cquasi-conductive,xe2x80x9d conduct electricity through the fabric and have surface resistivity of 109 to 1012 ohms per square and the sheath-core yarns are identified as xe2x80x9cquasi-conductivexe2x80x9d with a resistance of 108 ohms per meter. In order to attain the disclosed surface resistivity an antistatic coating is utilized. Without antistatic coating, the sheath-core yarns must be placed at a narrow spacing with the effective discharge area between the sheath-core yarns limited to 9 mm.
These patents teach against the use of conductive fibers in ungrounded antistatic applications. When relying upon the sheath-core yarns for static dissipation these fabrics are costly. In contrast, when relying on antistatic coating alone, such fabrics are susceptible to failure if the coating becomes removed during use. Additionally, when FIBCs comprise such fabrics are filled with non-conductive powders a surface charge potential of xe2x88x9232 kV (negative 32 kV) can be attained.
U.S. Pat. No. 5,071,699 to Pappas et al. discloses the use of conductive fibers in ungrounded antistatic fabric further comprising an antistatic coating. The resultant surface resistivity of the fabric is 1.75 times 1013 to 9.46 times 1013. When the coating is not present the disclosed fabrics do not adequately dissipate static charges. As a result, care must be taken to preserve the integrity of the coating.
The above patents are incorporated by reference. It is seen from the above that what is needed is a dissipative antistatic fabric that does not rely upon antistatic coatings or sheath-core filament yarns.
As a result, it is seen that a more robust anti-static textile fabric capable of preventing high surface charge levels is desirable, particularly a fabric that does not rely upon anti-static coatings or narrow spacing of quasi-conductor yarns.
In one embodiment, the present invention comprises a fabric with reduced electrostatic discharge energy suitable for ungrounded use in combustible atmospheres, clean room environments and flexible fabric containers. In some embodiments of the present invention, flexible fabric containers are constructed of the fabric and have reduced surface charge during filling operations of flexible fabric containers. The static dissipating fabrics of the present invention comprise fabric woven of non-conductive tapes, to which a plurality of conductive staple fibers are woven into or coated onto the fabric at a spacing of from 3 mm to 100 mm.